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Re: [TowerTalk] Fw: elevated radials vertical

To: "Gary Schafer" <>,"'towertalk'" <>
Subject: Re: [TowerTalk] Fw: elevated radials vertical
From: "Tom Rauch" <>
Reply-to: Tom Rauch <>
Date: Sun, 10 Jun 2007 18:52:03 -0400
List-post: <>
> What did you do to the ground system that made the base

> impedance go up but

> yet increased the field strength?

In the latest case we were testing elevated radials vs. 
buried radials with exactly the same radiator on 40 meters.

We moved the radiator higher or lower to match the height of 
the radials so the effective height stayed the same.

A small buried radial system producing 50 or 60 ohms base 
feedpoint impedance had slightly more FS than an elevated 
system using four radials. The elevated radial system was 
around the high thirty ohm range.

This is really pretty common. It is very common when the 
radials are longer than 90 degrees electrical. For example 
if I install a radial system with 180 degree radials the 
effect is often to increase the feedpoint impedance even 
though the FS does not decrease over the same number of 
shorter radials that have a low feedpoint impedance. I can 
also use ONE very short resonant radial with a very low ESR 
inductor and produce a ground with very low terminal 
impedance yet very poor efficiency.

There are multiple things that can throw a wrench into the 
idea what we measure at the feedpoint somehow tells us how 
the entire system behaves. The root problem is when a 
complex system has transmission lines with SWR impedances 
are transformed. This means what we read at one point has 
nothing to do with losses at points remote from the place we 
are measuring. Remember every conductor in the antenna 
system is to some extent a transmission line with surge 
impedance, even the radials.

The idea we can determine efficiency by watching the 
feedpoint is false when we try to determine distributed 
losses only at the feedpoint. The old Rradiation over 
Rground+Rradiation = efficiency is only a good formula when 
we understand the system and all losses and the radiation 
resistance is normalized to the feedpoint, but not when we 
don't or can't do that.

The only way I can think of to normalize losses to one point 
is to measure feedpoint resistance and then enclose the 
entire radiating system in a closed conductive dome in the 
farfield of the radiator, and then measure the change in 
resistance at that single point when we force all radiation 
to stop. This would normalize everything to the single 
point. Since few of us can do that, looking at the feedpoint 
is a waste of time.

If you want to see another example where people misapply 
ground losses, look at articles on mobile antennas. There 
are articles that conclude, because they only measure 
feedpoint resistance changes, some mounting spots on a small 
vehicle can produce ground losses of a couple ohms on 80 
meters. Good luck with that! Why install 50 or 60 1/4 wave 
radials when a $100 junk Toyota parked under the tower would 
be the same?

If we want to know the FS change, we need to measure the FS 
change. If we want to know ground losses we have to change 
the ground and watch FS. Measuring the feedpoint is not 

73 Tom


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